The foam that struck the space shuttle Columbia after lift-off and led to the deaths of all seven astronauts on board was defective, Nasa said on Friday.
An official investigation into the accident, conducted by the Columbia Accident Investigation Board, had left the matter open, since none of the foam or the fuel tank could be recovered for study.
But the space agency said on Friday that testing had since confirmed the defect and found the foam broke off the shuttle's external fuel tank because NASA did not know its procedures for applying foam insulation were flawed.
A suitcase-sized chunk of foam from an area of the tank known as the left bipod, one of three areas where struts secure the orbiter to the fuel tank during lift-off, broke off 61 seconds into the flight on Jan. 16 of last year. It gouged a large hole in Columbia's left wing.
The damage went undetected during the shuttle's 16-day mission, but caused the nation's oldest spacecraft to break apart under the stress of re-entering the Earth's atmosphere on Feb. 1, killing the astronauts.
"We now believe, with the testing that we've done, that defects certainly played a major part in the loss. We are convinced of that," said Neil Otte, chief engineer for the external tanks project. He spoke at the Michoud Assembly Facility in New Orleans, where the half-million pieces of every shuttle fuel tank come together.
The fault apparently was not with the chemical makeup of the foam, which insulates the tanks and prevents ice from forming on the outside when 500,000 gallons (1.9 million liters) of supercold liquid hydrogen and liquid oxygen are pumped aboard hours before lift-off.